1. Field of the Invention
The present invention relates to a device for detecting a foreign matter on a test piece for use in semiconductor production and, more particularly, on a photomask having a given pattern thereon.
2. Description of the Prior Art
Two important problems faced in improving semiconductor production processes in recent years have been improving product reliability and yields. Most defects found during the initial production processes in large-scale integration semiconductor (LSI and VLSI) device production are caused by foreign matter in the processes. Developing equipment that can detect such foreign matter at high speed and with high reliability is the key to overcoming this production problem.
Most conventional foreign matter detection devices are based on detection of laser scattered light, and are capable of detecting foreign matter with a minimum size of approximately 0.2 .mu.m using devices that scan mirror-finish test pieces. For example, U.S. Pat. No. 4,342,515 issued Aug. 3, 1982, to Akiba et al. (corresponding to the Japanese Laid-open Patent Publication No. 54-101390 published Aug. 9, 1979 (unexamined)). discloses a typical device which makes it possible to distinguish foreign matter from the pattern. Devices that scan test pieces with an imprinted pattern can detect foreign matter with a minimum size of approximately 0.5 .mu.m.
Referring to FIGS. 9A, 9B, and 9C, the operation of a conventional device capable of detecting foreign matter on a patterned test piece is described below. This device works by impinging a S-polarized laser beam 1 on the test piece having a pattern, and detecting differences in the polarization characteristics of scattered laser beams from the pattern edge and foreign matter.
FIG. 9A shows a case in which the S-polarized laser beam 1 is emitted approximately parallel to the upper surface of a photomask 6a having a pattern extending in a direction V, perpendicular to the S-polarized laser beam 1. As a result, there is no change in the polarization of reflected laser beam 2a from the photomask pattern 6a, and it enters into an object lens 5 with the S-polarization. Because a deflector plate 4 is placed to pass the light having a polarization perpendicular to the S-polarization, the reflected laser beam 2a is quenched by the deflector plate 4 and can not reach a detection element 3 such as a photoelectric multiplier.
FIG. 9B shows a case in which the S-polarized laser beam 1 is emitted to a photomask 6b in a manner very similar to that of FIG. 9A, but a pattern of photomask 6b extends in a direction V' having an angle .alpha. with respect to the direction V. The reflected laser beam 2b from the pattern of photomask 6b is not incident to the object lens 5, much less the detection element 3, as shown.
FIG. 9C shows a case in which the S-polarized laser beam 1 is emitted to a foreign matter 7 in a manner very similar to those of FIGS. 9A and 9B. When the laser beam 1 is impinged on the foreign matter 7, a P-polarized laser beam 2c having a polarization perpendicular to that of S-polarization is further reflected therefrom in addition to the S-polarized laser beam. Because the deflector plate 4 passes only the light having a polarization perpendicular to the S-polarization as described before, only the reflected laser beam 2c therefore can pass into the detection element 3 through the object lens 5 and deflection plate 4. Thus, the information with respect to the polarization in reflected laser beams 2a and 2c can be utilized to detect the foreign matter 7 on the photomasks 6a and 6b.
In general, when a laser beam is impinged on a test piece such as a photomask, non-directional scattered light is reflected from foreign matter on the photomask, and directional scattered light is reflected from the pattern formed on the photomask, specifically from the edge of the circuit pattern formed by a light shielded material such as chromium. Therefore, the foreign matter can be detected by utilizing the deflection or similar factors to shut the directional scattered light caused by the pattern edges.
However, in the conventional device with the above described configuration, when the laser beam 1 is impinged on a surface of the photomask, some portion of the laser beam 1 is reflected by the impinged surface of the photomask and the remaining portion thereof is incident to the photomask through the impinged surface. This reflected or incident laser beam reaches the detection element 3 after repeated reflecting or scattering inside the device, causing the element 3 to produce a photoelectric signal indicating that scattered light from foreign matter was received even though there is not actually any foreign matter present. This is particularly a problem when extremely minute foreign matter must be detected, or when the incidence angle of the deflected laser beam is offset from the horizontal. Thus, the conventional devices can not detect foreign matter with precision and reliability enough to satisfy the requirement of the present semiconductor production field.
Furthermore, in the conventional foreign matter detection device, it is impossible to recognize the shape and content or type of the foreign matter. This is because a photomultiplier can only process the photoelectric level of the reflected laser beam and can not produce the scanned area's image based on such a reflected laser beam. Therefore, it is necessary to move the laser beam 1 on the scanning area of the photomask to obtain the information with respect to the positional relationship between the detected foreign matter and the photomask, requiring substantial scanning time and a complicated mechanism for scanning.
Still furthermore, in the conventional foreign matter detection device, the incident angle of the S-polarized laser beam with respect to the surface of the test piece must be controlled within 2.degree. to 30.degree.. This is because when the incident angle is less than 2.degree., the pattern hinders the laser beam from impinging the neighboring pattern (such patterns are placed very closely together). Also when the incident angle is greater than 30.degree., the scanning area on the test piece impinged by the laser beam becomes too small.